Method of identifying a radio link

ABSTRACT

The invention relates to a Method of identifying a radio link  1,1′,1 ″ of a terminal device  2  to a wireless communication system ( 3 ). The method comprises first step of determining values of parameters of n≧2 radio links, whereby the parameters of each radio link are suitable to quantify at least an aspect of the quality of service of this radio link, the parameters (QoS parameters) being generic QoS parameters usable by different radio network technologies. In a second step a value of a metric is calculated, for each radio link, with the associated determined QoS parameters, whereby the value of the metric defines the overall quality of service of the corresponding radio link. In a third step one of the n≧2 radio links (best radio link) is identified as being the radio link offering the best quality of service by comparing the n≧2 values of the metric calculated in the second step. The method identifies a radio link which offers the best quality of service for a predetermined application in heterogeneous networks characterized by different radio access technologies.

TECHNICAL FIELD

The invention is based on a priority application EP 05291618.6 which ishereby incorporated by reference.

The invention relates to a method of identifying a radio link, wherebythe invention can be used in heterogeneous networks. In heterogeneousnetworks terminals may access a multitude of telecommunication systemsusing a multitude of radio access technologies (RATs). The inventionprovides a method and a corresponding computer program product withwhich the best radio link can be identified for the application theterminal is expected to provide. The computer program product can bepart of a terminal device or of a telecommunication communicationsystem, in particular a wireless communication system.

BACKGROUND OF THE INVENTION

The evolution of cellular mobile communication networks beyond the thirdgeneration and the introduction of new broadband wireless accesstechnologies such as WiMAX open the way to heterogeneous networks with adiversification of radio access technologies. In many cases providersoffering the same RAT compete with each other on the market.Furthermore, a provider may offer services of different RATs to a user.As an example, a provider may provide UMTS (universal mobiletelecommunications system) services as well as WLAN (wideband local areanetwork) services. In such a situation the user is not interested in theparticular technology, but is interested in getting the best quality ofservice (QoS) for his application. For that purpose it may be necessaryto carry out a handover to another radio link with or without a changeof the RAT.

In the prior art the desired application determines the RAT. If, forexample, a high data rate is desired, e.g. for downloading music or avideo file, WLAN is preferably used. If however, the user is interestedin video telephony, UMTS is a good choice.

Providing the best radio link for an application requires in many casesvertical handovers, i.e. handovers from a communication system operatingwith a first RAT to a communication system operating with a second RAT.However, each RAT has its own QoS specifications as it has differentmodulation scheme and access schemes. As an example, asignal-to-noise-interference (SNI) ratio measured in a system using afirst RAT, e.g. in a CDMA system, can hardly be compared with a SNI in asystem using a second RAT, e.g. when a TDM link is established. As aconsequence, the radio link quality in the new system cannot beanticipated and providing the best QoS for the user becomes difficult.

In the document A. Festtag, “Optimization of handover performance bylink layer triggers in IP-based networks: parameters, protocolextensions and APIs for implementation”, TKN technical reportTKN-02-014, TU Berlin, version 1.1, August 2002, two phases of ahandover process are identified: a handover detection and triggeringphase and a handover execution phase. In order to speed up the firstphase the authors suggest the definition of a link layer trigger for ahandover. A parameter for this link layer trigger is an abstract measureof a signal quality. This abstract measure is obtained by a mapping ofRAT-specific measurement values into this abstract measure.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method ofidentifying a radio link which offers the best quality of service, inparticular for a predetermined application.

It is another object of the invention to provide a method of selecting aradio link which improves network efficiency and load distribution inthe communication system.

It is still another object of the invention to provide a computerprogram product, a terminal and a communication system applying themethod.

This object and other objects are solved by the features of theindependent claims. Preferred embodiments of the invention are describedby the features of the dependent claims. It should be emphasized thatany reference signs in the claims shall not be construed as limiting thescope of the invention.

The underlying principle of the invention is to use generic QoSparameters, which means RAT-independent parameters, to characterize thequality of a radio link. The generic QoS parameters may thus be used bysystems using arbitrary RATs such as GSM, UMTS, WiMAX, WLAN, Bluetoothetc.

Each radio link is characterized by a set of such generic QoSparameters. The radio links might be associated with the same RAT. Anexample would be a situation in which a mobile terminal has access todifferent base stations belonging to the same communication system andusing the same RAT such as UMTS. In the alternative the radio links maybe associated with different RATs such as UMTS, WLAN, WiMAX etc.

Each generic QoS parameter is suitable to quantify at least an aspect ofthe quality of service of a radio link. The set of generic QoSparameters then serve to characterize the overall radio link quality.The generic QoS parameters may be identical to the QoS parameters knownfrom UMTS or WLAN and may especially include:

-   -   the mean and peak data rate (kbit/s)    -   the packet delay (ms)    -   the delay jitter (ms)    -   the maximum packet loss rate (%) or the bit error rate or the        block/frame error rate (per ‰).

A more comprehensive list of QoS parameters are listed in Table 1 forthe case of UMTS systems. These are the radio access bearer attributesas defined in 3GPP document TS 23.107 V.6.1.0 and TS 25.413 V.6.0.0.Table 2 shows the QoS parameters for WLAN systems according toIEEE802.11e.

However, the QoS parameters may differ from the QoS parameters knownfrom UMTS in order to take considerations of other RATs into account.This modified set of generic QoS parameters may be agreed upon in astandardization process. In the following description QoS parameterswill always be generic QoS parameters unless otherwise indicated.

In a first step of the method generic QoS parameters are determined.More precisely, all QoS parameters for each of n≧2 radio links aredetermined. n is an integer. This determination can be done by aterminal device and/or by a wireless communication system as will bedescribed below in more detail.

In a second step the multitude of QoS parameters of each radio link aremapped into a single value by means of a metric. A metric shall beunderstood to be a mathematical function having the multitude of QoSparameters as variables, whereby the metric calculates a value having areduced number of variables. The output of the metric may be a matrix, avector, or a scalar value. The value of the metric is calculated foreach of the n≧2 radio links. The values obtained with the metric definethe overall quality of service of each of the n≧2 radio links, such thatthe calculation serves to acquire values with which the overall radiolink quality can be compared in an objective way.

Quantifying the quality of a radio link by means of the values of themetric then makes it possible to identify one of the n≧2 radio links ina third step as being the radio link offering the best overall qualityof service. Consequently, the identified radio link is called the “bestradio link”. The third step thus consists of a simple comparison of then≧2 values obtained in the second step.

The advantage of generic QoS parameters as used by the method disclosedabove is that, if they are delivered to layer 3 or 3.5 or to a genericlink layer on layer 2.5, they can be directly compared between two ormore RATs. This means that systems operating with different RATs caneasily interpret the QoS parameters of systems with another RAT as theyuse the same parameters, or simply speaking because they use the samelanguage. This is however a prerequisite for anticipating aspects of theradio link quality for possible vertical handovers.

The use of the metric provides the advantage that it is possible toidentify the best radio link for a terminal device. This is the basisfor handover decisions in heterogeneous networks as it facilitates ahandover to the best cell. As it will be described below in more detailit is also possible to choose a metric to identify the best radio linkfor a particular application running on the terminal device. As anexample, it is possible to identify the best radio link for videostreaming or video telephony. This is however the primary interest of auser which can thus be satisfied more easily.

Furthermore, it has the potential for increasing the network efficiencyand the load distribution. Therefore, the metric to identify the “bestradio link” is extended to include QoS parameter on the consumption ofsystem resources, so that more efficient radio links, e.g. with a higherbit rate per bandwidth for the particular application, are preferred.

There are quite a few possibilities of determining generic QoSparameters. One possibility is to measure and/or estimate QoS parameterswhich are specific for a particular RAT and to map or transform theminto generic QoS parameters. As an example, UMTS-specific QoS parametersmay be mapped into generic QoS parameters. This means that existingalgorithms for determining RAT-specific QoS parameters can still beused, and that only an additional piece of software is necessary to mapthe RAT-specific QoS parameters into generic QoS parameters. The mappingcomprises information which is specific to only one single RAT, and isthus fully within the scope of OSI layer 2.

A second possibility of determining generic QoS parameters is to measureand/or estimate them directly. This is useful for a communication systemoffering services with several RATs. In this case the computer programfor the determination of the QoS parameters is simplified as only asingle algorithm is necessary instead of a multitude of algorithms foreach RAT.

Another possibility of determining generic QoS parameters consists inreceiving them. If it is the terminal device which determines the QoSparameters it may receive further generic QoS parameters from thecommunication system. In the alternative the mobile terminal may receivefurther RAT-specific QoS parameters from the communication system formapping or transforming them into generic QoS parameters. The receptionof parameters by the terminal is advantageous when specific or genericQoS parameters can't be determined by the terminal device itself, forexample statistical data on medium usage. Furthermore, the received QoSparameters may be parameters from system information data bases whichalso contribute to the quality of service such as

-   -   the type of the application    -   capabilities such as hardware features and supported protocol        options and access information    -   operator policy    -   network load for a load balancing between RATs and networks    -   the supported mobility in the RAT, e.g. a typical cell radius or        a maximum tolerated user velocity    -   the signaling load and latency for the handover,    -   the cost of link operation    -   the security level    -   dedicated access rights, e.g. in a non-public area of the        network or in areas with restricted access    -   user subscription restrictions and user preferences the latter        e.g. due to connection costs.

If it is the communication system which determines the QoS parameters itmay receive further generic QoS parameters from the terminal device. Inthe alternative the communication system may receive furtherRAT-specific QoS parameters from the terminal device for mapping ortransforming them into generic QoS parameters. QoS parameters to bereceived by the communication system may be

-   -   the type of the application    -   measured QoS parameter such as link budget or data rate    -   capabilities such as hardware features and supported protocol        options    -   the user position or velocity    -   the time delay to switch between RATs (zero indicating parallel        usability of both RATs in the terminal,    -   dedicated access rights such as stored on a SIM card    -   user preferences due to connection costs

Instead of receiving the QoS parameters themselves it is also possibleto receive only mathematical representations of them. The mathematicalrepresentation might be an intermediate value which can be inserted intothe metric to calculate the radio link quality. The exact nature of thisintermediate value depends strongly on the metric which is used. As anexample, the communication system may determine 15 generic QoSparameters and may send an intermediate value representing these 15parameters. The terminal device determines 10 generic QoS parametersitself, and uses them together with the intermediate value to calculatethe value of the metric. As a further example, this approach is possiblewhen the metric consists of the product of all QoS parameters. In thiscase the mathematical representation is the product of theabove-mentioned 15 QoS parameters. This product is then multiplied withthe 10 QoS parameters determined by the terminal device.

This approach has the advantage to reduce the signalling load.

As a matter of fact, it is not only possible to receive mathematicalrepresentations of generic QoS parameters, but also mathematicalrepresentations of RAT-specific QoS parameters. In this case themathematical representation might be an intermediate value usable in thealgorithm which maps the RAT-specific QoS parameters into generic QoSparameters.

After determining QoS parameters they are used for calculating a valueof the metric. This may basically be done by an arbitrary combination ofdetermined QoS parameters. If the communication system carries out thecalculation, it may calculate the metric with generic QoS parametersreceived from the terminal device and with generic QoS parametersdetermined by itself. Correspondingly, if the terminal device carriesout the calculation, it may calculate the metric with generic QoSparameters received from the communication system and generic QoSparameters determined by the terminal device.

In a preferred embodiment of the invention the values of the metric arescalar values. In this case comparing the quality of different radiolinks is particularly easy. As an example, a large value of the metricmight indicate a link with good overall quality of service for anapplication, whereas a low value would indicate a link with bad overallquality of service.

In another preferred embodiment of the invention the metric is aconfigurable metric. Configurable means that parameters, namelyparameters of the metric which are not QoS parameters, can be adjustedsuch that the terminal device or the communication system may adapt themetric to the specific situation, e.g. to the type of the application orto the operator policy. Furthermore, the configuration may be used toadjust the metric to a situation with limited availability of the QoSparameters, e.g. to calculate the metric results using only the QoSparameters it is able to determine but not those it would need toreceive.

A configurable metric has the advantage of reducing the signallingbetween the terminal device and the communication system. The reason isthat the configurable metric allows identifying radio links showing anunacceptably low radio link quality. As an example, the QoS parametersdetermined by a terminal device may indicate clearly that the radio linkis unsuitable for internet browsing because the data rate is too low. Insuch a case the QoS parameters of these radio links are not transmittedto the communication system or the terminal device respectively asfurther calculations are unnecessary. This reduces the overhead byavoiding a transmission of QoS parameters, or mathematicalrepresentations of them, which will not be used for a future accessanyway.

In a preferred embodiment the method includes the step of calculating avalue of a metric for a multitude of radio links, and of creating aquality ranking of the multitude of radio links on the basis of theirvalues of the metric. This quality ranking can be used for handoverdecisions to identify the radio links which offer a better quality ofservice as the current serving radio link. This can be used to decreasesignalling even further as a transmission of QoS parameters over the airinterface becomes only necessary when they indicate that the rankingmight have changed. As a matter of fact, the quality ranking itself canbe transmitted from the communication system to the terminal device orvice versa.

In a preferred embodiment of the invention the metric is calculatedaccording to the formula$\prod\limits_{i}\left( \frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)} \right)^{a_{i}}$QoS_(determined)(i) are the determined QoS parameters as explainedabove.QoS_(application)(i) are the corresponding QoS parameters as requestedby an application running on the terminal device.i is an integer ranging from 1 to N, whereby N is the number ofdetermined QoS parameters. a_(i) are weighting factors.

In general QoS_(application)(i) are maximum allowed values or minimumallowed values of generic QoS parameters for a particular application.Examples for a conversational application would be

-   -   a delay in the MAC layer of less than 50 ms    -   a jitter of less than 5 ms    -   a bit-error-rate of less than 1%

For browsing in the internet QoS_(application)(i) might be

-   -   a delay in the MAC layer of less than 150 ms    -   a data rate of more than 1 MBit/s    -   a bit-error-rate of less than 0.1%

With the weighting factors a_(i) the metric becomes a configurablemetric, with which the metric can be adapted to the specific applicationrunning on the terminal device. The reason is that each applicationrequires a specific set of QoS parameters. As an example, a MAC layerdelay will have a higher weighting for a real-time application than fora background service. In the latter case the corresponding exponenta_(i) might be set to one or even to zero. The weighting factors thusallow the determination of an application-specific link quality, andthus of a link quality measure which is specifically tailored to thedemand of the user or to the operator. It might however be necessary tofind a certain compromise value for these weighting factors in the casethat the user uses different applications at the same time.

In a further embodiment the metric is calculated according to${\prod\limits_{i}\left( {f_{i}\left( x_{i} \right)} \right)^{a_{i}}},$whereby f(x_(i)) is a mathematical function with$x_{i} = {\frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)}.}$An example of the function f(x_(i)) will be a clip function as will bediscussed below.

The calculation of the quality metric as disclosed above may yieldunreasonable values in those cases in which one of the QoS parametershas an unacceptably low value, e.g. a value which is below a predefinedthreshold value. This can be compensated by complementing the metricwith a clipping function, whereby the QoS component having thisunacceptably low value is clipped to zero. This can be done by operatinga clip function on some or all of the QoS ratios in the metric. Thisapproach sets the whole metric to zero, such that this radio link willbe disregarded in the quality ranking mentioned above. This avoids thata low measurement value of a first QoS parameter can be compensated by ahigh measurement value of a less important second QoS parameter.

Correspondingly, an over-provisioning of the QoS parameter may beclipped to a maximum value in the metric. In this case a QoS parameterin the metric shall not regard to have a value which is higher than therequested value. An example would be a radio link offering a data rateof 54 MBit per seconds for a video streaming which requires only 384kBit/s. In this case the corresponding value of QoS_(determined)(i) isset to 384 kBit/s for all those values which are larger than 384 kBit/s.

When the surmounting data rate doesn't further increase the term in themetric all links with sufficient bandwidth are counted equivalently.Thus, the clipping retains the significance of other parameters likedelay or loss. In this case handovers can be restricted to those casesin which the target cell offers a new useful QoS improvement for theapplication.

According to a preferred embodiment of the invention each factor of theproduct of the metric is modified by a clip function${{clip}_{i}(x)} = \left\lbrack {{\begin{matrix}0 & {for} & {x_{i} < \min_{i}} \\x_{i} & {for} & {\min_{i}{< x_{i} < \max_{i}}} \\\max_{i} & {for} & {\max_{i}{< x_{i}}}\end{matrix}\quad{with}\quad x_{i}} = \frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)}} \right.$whereby min_(i) represents a minimum allowed value and max_(i) a maximumuseful value of the quantity x_(i). Using this clipping function can bedone by the terminal device as well as by the communication system.

In a further preferred embodiment the weighting factors a_(i) areupdated from time to time and transmitted from the communication systemto the terminal. This takes into account that the weighting factors willin most cases be operator-specific and subject to his policy, and may bechanged from time to time to reflect changes of this policy. In such acase the determination of the parameters of the n≧2 radio links may becarried out by the terminal device which calculates the values of themetric after having received the weighting factors a_(i) from thecommunication system.

In a further preferred embodiment the terminal device transmits atrigger to the communication system if the serving radio link is not thebest radio link. In this case the best radio link, or a radio link whichis better than the serving radio link according to the quality ranking,can be selected as a future serving radio link for a handover. This isespecially useful when the handover decision is network based but shallfor load reasons be decided not after each measurement but only in caseof relevant events. Only when the terminal detects (by the use of itsconfigured metric) such an event, it transmits a trigger signal to thenetwork decision function. This trigger may include all or selectedmeasured QoS parameter and a proposed handover target. The network thentakes the final decision on the target link and on the proper timing forcarrying out a handover.

It goes without saying that the method as disclosed above can beimplemented in a terminal device and in a wireless communication systemrespectively, such that the invention can be carried out by a computerprogram. This computer program might be stored on the appropriatestorage medium such as the CD or DVD, or may be transmitted by means ofelectrical carrier signals over a network such as the Internet.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments thereafter. It should benoted that the use of reference signs shall not be construed as limitingthe scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES

FIG. 1 shows a communication system using the invention,

Table 1 lists QoS parameters for UMTS systems.

Table 2 lists QoS parameters for WLAN systems.

Table 3 lists generic QoS parameters as requested by an application,

Table 4 lists generic QoS parameters as determined by a mobile terminal,

Table 5 lists exponents of the quality metric,

Table 6 lists values of the metric with clipping,

Table 7 lists values of the metric without clipping.

DETAILED DESCRIPTION OF THE DRAWINGS AND OF THE PREFERRED EMBODIMENT

The only figure shows a communication system 1 which is using theinvention. A mobile terminal 2 may establish radio links 5, 5′, 5″ tobase stations 6, 6′ and 6″ respectively. Base station 6 is a node Boffering UMTS services, base station 6′ is a node B offering UMTS andhigh speed downlink packet access (HSDPA) services, and base station 6″is a hot spot offering WLAN services. Each base station 6, 6′ and 6″ isin communication with a corresponding radio resource controller 7,7′,7″,each of which comprises a radio resource management system 8, 8′,8″. Forsake of simplicity base stations 6, 6′ and 6″ belong to the sameprovider.

A system component 4 of the communication system 1 is connected with afibre optic 10 to RNC 7″. It includes an optical disk drive 9 forinsertion of DVDs 3.

The provider has the possibility to transmit a metric from systemcomponent 4 via fibre optic 7, RNC 7″ and hot spot 6″ to terminal 2,namely the metric $\begin{matrix}{f = {\prod\limits_{i}\left( \frac{{QoS}_{measured}(i)}{{QoS}_{application}(i)} \right)^{a_{i}}}} & \left( {{equation}\quad 1} \right)\end{matrix}$

The nature of the factors of this metric have been described above.

Table 3 shows QoS parameters as requested by an application of theterminal 2, this means QoS_(application), for three differentapplications, namely video telephony, video streaming and a musicdownload.

Table 4 shows the QoS parameters as measured by the terminal 2 at anexample channel loss of one or two percent respectively. The metric,which is used for a calculation of the quality of the radio links is asfollows: $\begin{matrix}{f = {\left( \frac{available\_ rate}{mean\_ rate} \right)^{r}*\left( \frac{actual\_ delay}{max\_ delay} \right)^{d}*\left( \frac{actual\_ loss}{max\_ loss} \right)^{l}}} & \left( {{equation}\quad 2} \right)\end{matrix}$

Table 5 lists the exponents a₁=r, a₂=d, and a₃=l as used in the metric.The terminal 2 receives the metric of equation 2 and the parametersQoS_(application)(i), i={1,2,3}, from the communication system 1, andstores them in its internal memory (not shown). It regularly measuresthe above-mentioned generic parameters directly.

Table 6 and 7 show the values of the metric. In the case of table 6 aclipping has been performed for those cases in which a factor ofequation 2 is larger than 1. In this case the factor has been clipped to1.

As can be derived from table 6 the best radio link for video telephonyis access point 5 offering UMTS services only, access point 5′ offeringUMTS and HSDPA services is the best base station for video streaming,and hot spot 5″ is the best base station for music downloads.

Table 7 shows the corresponding values of the quality metric in thosecases in which no clipping has been performed. In the case of node B 5offering UMTS services and servicing the terminal 2 with video streaminga very large number is obtained, namely 33,33. This is a high rankingeven though the data rate is rather poor. The reason is that there is anoverprovision of the delay requirement of a factor of more than 200.

LIST OF REFERENCE NUMERALS

-   -   01 wireless communication system    -   02 terminal    -   03 computer readable medium    -   04 system component    -   05 radio link    -   05′ radio link    -   05″ radio link    -   06 node B    -   06′ node B    -   06″ hot spot    -   07 radio resource controller    -   07′ radio resource controller    -   07″ radio resource controller    -   08 radio resource management (RRM) system    -   08′ radio resource management (RRM) system    -   08″ radio resource management (RRM) system    -   9 optical disk drive

10 cable TABLE 1 Conver- Back- sational Streaming Interactive groundTraffic class class class class class Maximum bitrate X X X X Deliveryorder X X X X Maximum SDU size X X X X SDU format information X X SDUerror ratio X X X X Residual bit error ratio X X X X Delivery oferroneous X X X X SDUs Transfer delay X X Guaranteed bit rate X XTraffic handling priority X Allocation/Retention X X X X priority Sourcestatistics descriptor X X Signalling Indication X

TABLE 2

TABLE 3 Video Video telephony streaming Music download QoS application(real time) 10 sec buffered (50 MByte File) Minimum data rate 16kBit/sec 16 kBit/sec 384 kBit/sec Mean data rate 128 kBit/sec 384kBit/sec 3000 kBit/sec Maximum data rate 384 kBit/sec 1000 kBit/sec30000 kBit/sec Maximum delay 0.1 sec 10 sec 10 sec Delay variance 0.01sec 5 sec 5 sec Maximum packet 5% 1% 10% loss rate

TABLE 4 QoS measured UMTS UMTS HSDPA WLAN Maximum data rate 384 kBit/sec2000 kBit/sec 54000 kBit/sec Available data rate 64 kBit/sec 384kBit/sec 2000 kBit/sec Available delay 0.05 sec 0.2 sec 2 sec Availabledelay variance 0.01 sec 0.01 0.01 sec 1 sec Available packet loss rate1% 1% 2%

TABLE 5 Application Video telephony Video streaming Music download r 1 12 d −2 −1 0 l −1 −1 −1

TABLE 6 with clipping UMTS UMTS HSDPA WLAN Video telephony 0.50 0.250.01 Video streaming 0.17 1.00 0.5 Music download 0.00 0.02 0.44

TABLE 7 without clipping UMTS UMTS HSDPA WLAN Video telephony 10.00003.75 0.10 Video streaming 33.3333 50.00 13.02 Music download 0.004550.16 2.22

1. A method of identifying a radio link of a terminal device to awireless communication system, the method comprising the followingsteps: a) determining values of parameters of n≧2 radio links, wherebythe parameters of each radio link are suitable to quantify at least anaspect of the quality of service of this radio link, the parameters (QoSparameters) being generic QoS parameters usable by different radioaccess technologies, b) calculating, for each radio link, a value of ametric with the associated determined QoS parameters, whereby the valueof the metric defines the overall quality of service of thecorresponding radio link, c) identifying one of the n≧2 radio links(best radio link) as being the radio link offering the best quality ofservice by comparing the n≧2 values of the metric calculated in step b),wherein determining values of generic QoS parameters is done bymeasuring and/or estimating them, or measuring and/or estimating QoSparameters specific to a radio access technology, and mapping theobtained values into generic QoS parameters.
 2. The method according toclaim 1, wherein determining values of generic QoS parameters is done byreceiving generic QoS parameters, or mathematical representations ofgeneric QoS parameters, or receiving RAT-specific QoS parameters andmapping them into generic QoS parameters.
 3. The method according toclaim 1, wherein calculating the metric is done with generic QoSparameters received from the terminal device and generic QoS parametersdetermined by the communication system, and/or with generic QoSparameters received from the communication system and generic QoSparameters determined by the terminal device.
 4. The method according toclaim 1, wherein the output values of the metric are scalar values. 5.The method according to claim 1, wherein the output values of the metricare scalar values, and that larger values indicate a higher quality ofservice of the associated radio link.
 6. The method according to claim1, wherein it further comprises the step of creating a quality rankingof the n≧2 radio links on the basis of the n≧2 values of the metric. 7.The method according to claim 1, wherein the metric is calculatedaccording to$\prod\limits_{i}\left( \frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)} \right)^{a_{i}}$ whereby QoS_(determined)(i) are the determined values of the genericQoS parameters, whereby QoS_(application)(i) are values of the genericQoS parameters as requested by a particular application running on theterminal device, and whereby a_(i) are weighting factors.
 8. The methodaccording to claim 1, wherein the metric is calculated according to${\prod\limits_{i}\left( {f_{i}\left( x_{i} \right)} \right)^{a_{i}}},$ whereby${x_{i} = \frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)}},$ whereby QoS_(determined)(i) are the determined values of the genericQoS parameters, whereby QoS_(application)(i) are values of the genericQoS parameters as requested by a particular application running on theterminal device, whereby f_(i)(x) are mathematical functions and wherebya_(i) are weighting factors.
 9. The method according to claim 8, whereinthe functions f_(i)(x) are clip functions of the type${{clip}_{i}(x)} = \left\lbrack {{\begin{matrix}0 & {for} & {x_{i}\quad < \quad\min_{i}} \\x_{i} & {for} & {\min_{i}{< x_{i} < \max_{i}}} \\\max_{i} & {for} & {\max_{i}{< x_{i}}}\end{matrix}\quad{with}\quad x_{i}} = \frac{{QoS}_{determined}(i)}{{QoS}_{application}(i)}} \right.$ whereby mini represents a minimum allowed value and max_(i) a maximumallowed value of the quantity x_(i).
 10. The method according to claim9, wherein determining the parameters of the n≧2 radio links is carriedout by the terminal device, and that the terminal device calculates thevalues of the metric after having received the weighting factors a_(i)from the communication system.
 11. The method according to claim 6,wherein the terminal device transmits a trigger to the communicationsystem if the serving radio link is not the best radio link, to coursethe communication system to consider or decide a handover to a betterradio link as a future serving radio link.
 12. A computer programproduct for identifying a radio link, the computer program productcomprising a computer readable medium, having thereon computer programcode means, when said program is loaded, to make a computer executablefor carrying out the method according to claim
 1. 13. A terminal devicefor accessing a wireless communication system, wherein comprising acomputer program product according to claim
 12. 14. A wirelesscommunication system, wherein comprising a computer program productaccording to claim 12.